Flat panel type radiation detectors (also denoted simply as FPD), which have been developed as a digital radiation imaging technology capable of obtaining digital radiation image directly, include a scintillator system in which radiation is converted to a visible light through a phosphor such as Gd2O2S or CsI, followed by conversion to an electric charge through photodiode, and a system of converting X-rays directly to an electric charge through an X-ray detecting element, as typified by Se. The present invention is related to a flat panel detector of the former scintillator system. Examples of a flat panel detector of a scintillator system include an FPD in which a scintillator panel is combined with a photoelectric conversion device of a thin layer transistor (TFT) and a charge coupled device (CCD), as described in JP 2005-114556 A.
Recently, there were commercialized, flat panel detectors using CCD, CMOS or the like, for use in industrial nondestructive examination or for dental to be inserted in the mouth. Specifically development for use in dental diagnosis has been remarkable, resulting in replacement with film in a broad range including a small device for use in the mouth, panoramic photographing and cephalometric analysis (as described in, for example, JP 2004-105518 A). As a result of such a variety of sizes or forms, enlargement of effective image area or flexibility of are required for SA scintillator panels.
A phosphor capable of converting radiation to visible light requires use of a phosphor exhibiting enhanced emission efficiency to achieve an enhanced SN ratio even when photographed at a relatively low dosage. The emission efficiency of a scintillator panel is generally dependent on the phosphor layer thickness or X-ray absorption coefficient. However, an increased thickness of a phosphor layer causes scattering of emitted light within the phosphor layer, leading to lowering of image sharpness. Accordingly, defining sharpness required for image quality determines a specific layer thickness.
Specifically, cesium iodide (CsI) exhibits enhanced conversion efficiency of X-rays to visible light and can easily form a phosphor of a columnar crystal structure through vapor deposition, whereby scattering of emitted light within a crystal is inhibited through a light guide effect, rendering it feasible to increase the thickness of a phosphor layer. However, the CsI alone exhibits lowered emission efficiency, so that a mixture of CsI and sodium iodide (NaI) at an appropriate ratio is deposited on a support in the form of a sodium activated cesium iodide (CsI:Na) through vapor deposition, as described in, for example, JP 54-035060 B. Alternatively, recently, a mixture of CsI and thallium iodide (TlI) at an appropriate ratio is deposited on a support in the form of a thallium activated cesium iodide (CsI:Tl) through vapor deposition, followed by annealing to achieve enhanced visible light conversion efficiency, which is employed as an X ray phosphor.
A phosphor comprising CsI as a parent material exhibits deliquescence and exhibits such a defect of characteristics being deteriorated with aging. Accordingly, there was proposed formation of a moisture-proofing protective layer on the surface of a phosphor layer employing CsI to inhibit such deterioration with aging. For instance, there were disclosed a technique of covering the upper portion and the side portions of a phosphor layer and the circumference of a support with a polyxylylene resin (as described in, for example, JP 2004-105518 A); and a technique of covering at least the opposite surface of the side opposed to a support and the side surface with a transparent resin film exhibiting a moisture permeability of less than 1.2 g/m2·day (as described in, for example, JP 2005-308582 A). These protective layers can achieve enhanced moisture resistance.
Durability of such a moisture-resistant protective layer leads to durability of a scintillator layer as such and there are known a technique of covering the overall surface of a support together with the scintillator with poly(p-xylylene), as described in, for example, JP 2002-116258 A, a technique of providing a concave-convex surface to a support to prevent poly(p-xylylene) from peeling (as described in, for example, JP 2005-338067 A), and, in case of a transparent resin film, a technique of melting the edge portions of a phosphor to prevent the interior of the film from being damaged by the phosphor edge, as described in, for example, JP2008-139291 A. However, coverage of the overall surface or providing a concave-convex to a support results in increased cost or makes production prove complicated, leading to increased load, which is not satisfactory.
As a method of producing a radiation image conversion plate comprising a support and a phosphor layer, there is known a method of cutting a radiation image conversion plate by exposing a phosphor layer (on the opposite side of a support) to a laser light (as described in, for example, JP 2008-213043 A). This is not a production method of a scintillator panel but application of this method to a scintillator panel resulted in problems such that adhesion of a protective layer to a phosphor layer was lowered, leading to deterioration in moisture resistance.